1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device, an image forming apparatus, and a fixing method, and more particularly, to a fixing device for fixing a toner image on a recording medium, an image forming apparatus including the fixing device, and a fixing method for fixing a toner image on a recording medium.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically foam an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may employ a fixing roller and a pressing roller pressed against the fixing roller to form a nip therebetween through which the recording medium bearing the toner image passes. As the recording medium passes through the nip, the fixing roller heated by a heater and the pressing roller together apply heat and pressure to the recording medium, thus melting and fixing the toner image on the recording medium.
FIGS. 1 and 2 are schematic vertical sectionals of a fixing device 15R having the above-described configuration. As illustrated in FIG. 1, a pressing roller 61 is pressed against a fixing roller 63 heated by a heater 62. As a recording medium 69 bearing a toner image passes through a nip N formed between the pressing roller 61 and the fixing roller 63, the toner image is fixed on the recording medium 69 by heat and pressure from the fixing roller 63 and the pressing roller 61.
If the fixing roller 63 overheats the recording medium 69, moisture contained in the recording medium 69 is vaporized, causing two related problems. A first problem is adherence of water droplets to the unfixed toner image on the recording medium 69, producing a spotty or other faulty toner image. A second problem is a weakening of the rigidity of the recording medium 69, causing the recording medium 69 to warp and producing a distorted or other faulty toner image.
The first problem of adhering water droplets to the unfixed toner image on the recording medium 69 arises when vaporized moisture adhering to the interior of the fixing device 15R moves to the unfixed toner image on the recording medium 69. Several solutions to this problem have been proposed, such as employing hygroscopic materials in the components disposed in proximity to the path along which the recording medium 69 is conveyed (recording medium conveyance path), passing a blank sheet through the nip N formed between the fixing roller 63 and the pressing roller 61 before the fixing operation to absorb water droplets, driving a heater and a fan based on the temperature and humidity measured inside and outside the image foaming apparatus, and installing a dehumidifier heater in a paper tray that stores recording media to be supplied to the fixing device. However, such solutions increase both the number of parts and consumption of blank sheets and power, resulting in increased manufacturing costs and upsizing of the image forming apparatus while adverse affecting the environment.
The second problem of warping the recording medium arises when the recording medium loses its rigidity due to vaporization of moisture contained therein. For example, as shown in FIGS. 1 and 2, a guide assembly 60 constructed of an exit guide 41, a swing guide 42, and a conveyance guide 43 may be disposed downstream from the fixing device 15R and upstream from an output roller pair 45 in the conveyance direction of the recording medium 69. As shown in FIG. 1, the rigid recording medium 69 conveyed from the nip N formed between the pressing roller 61 and the fixing roller 63 to the output roller pair 45 contacts the swing guide 42 and is guided by the swing guide 42 to the output roller pair 45 without being warped. By contrast, as shown in FIG. 2, the recording medium 69 with a decreased rigidity due to vaporization of moisture contained therein may be warped toward the pressing roller 61 when discharged from the nip N, and then may strike the exit guide 41 and the swing guide 42 in a state in which the leading edge of the recording medium 69 is nipped and pulled by the output roller pair 45, thus warping the recording medium 69. When the fixing roller 63 and the pressing roller 61 apply heat and pressure to the unfixed toner image on the warped recording medium 69, the toner image may be distorted.
Referring to FIG. 3, a detailed description is now given of the mechanism whereby vaporization of moisture contained in the recording medium 69 takes place.
FIG. 3 is a timing chart showing a temperature waveform T of a known method of controlling the temperature of the fixing roller 63. The image forming apparatus may provide a high definition mode that forms a high definition toner image by heating the toner image on the recording medium 69 conveyed through the nip N for a longer time at a decreased speed. Since the fixing roller 63 heats the toner image for the longer time, the toner image needs to be fixed at a temperature of the fixing roller 63 that is equivalent to a target fixing temperature T1 lower than a target standby temperature T0. However, if the fixing operation starts before the temperature of the fixing roller 63 decreases from the target standby temperature T0 to the target fixing temperature T1, the fixing roller 63 overheats the recording medium 69, vaporizing the moisture contained in the recording medium 69. For the reasons described above such vaporization is undesirable, and accordingly, there is a need for a technology to prevent vaporization of moisture from the recording medium 69.